1. Field of the Invention
The invention relates to methods and apparatus for overcladding a glass rod. More particularly, the invention relates to Rod-In-Tube (RIT) methods and apparatus for overcladding a glass rod.
2. Description of the Related Art
Optical fibers typically are fabricated by heating and drawing a portion of an optical preform comprising a refractive core surrounded by a protective glass cladding. Conventionally, several processes exist for fabricating preforms, including a modified chemical vapor deposition (MCVD) process. See, e.g., U.S. Pat. No. 4,217,027, which is issued to MacChesney et al. on Aug. 12, 1980 and co-owned with this application. Another conventional process is the vapor axial deposition (VAD) process.
In the MCVD process, precursor gases such as SiCl4 and GeCl4 pass through a rotating substrate tube of silica glass. A torch heats the tube from the outside as the precursor gases pass therethrough, causing deposition of submicron-sized glass particles on the inside surface of the tube. Movement of the torch along the longitudinal axis of the tube in a plurality of passes builds up layer upon layer of glass to provide a preform tube. Once a suitable number of layers have been deposited, the preform tube is heated to cause it to collapse into a solid rod typically referred to as the preform, the preform rod, the core rod or the preform core rod.
The preform core rod then is inserted into a glass overclad tube, which is collapsed onto the preform core rod using heat and a pressure gradient present about the overclad tube. Such process typically is referred to as the Rod-In-Tube (RIT) process. See, e.g., U.S. Pat. No. 4,820,322, which is co-owned with this application, and hereby is incorporated by reference herein. Depending on the desired final size of the optical preform, steps of the RIT process are repeated. More specifically, to produce optical fiber preforms having larger diameters, e.g., 63 millimeters (mm) or larger, the RIT process is performed twice. That is, a first overclad tube is collapsed onto the preform core rod using a conventional RIT process and then a second overclad tube is collapsed onto the existing overclad preform rod using a second RIT process. The resulting doubleclad optical fiber preform is desirably larger than preforms made with a more conventional, single RIT overcladding process. Typically, preforms having larger diameters tend to yield more optical fiber drawn therefrom than smaller preforms. The increased yield thus improves productivity of preform manufacturing processes, as the number of preforms used per length of optical fiber is reduced, as are the associated setup and installation times.
Other processing variations exist for manufacturing optical fiber preforms. For example, in conventional RIT processes, the collapse of the overclad tube onto the preform core rod typically is performed while the overclad tube and the preform core rod are mounted in a vertical lathe. Alternatively, the collapse of the overclad tube on the preform core rod is performed in a draw tower furnace, which also is used to draw optical fiber from the resulting optical fiber preform. Such collapse is accomplished by inserting the preform core rod into an overclad tube and then moving the combined preform core rod and overclad tube coaxially through the draw tower furnace, which causes collapse of the overclad tube onto the preform core rod prior to the drawing of the fiber. Such technique often is referred to as Overclad During Draw (ODD).
Although existing RIT and ODD processes and devices save process time (and money), further efficiencies in MCVD optical fiber manufacturing processes are desired. For example, it would be desirable to have available methods and apparatus for manufacturing larger overclad preforms without involving additional RIT process overcladding steps. Similarly, for example, it would be desirable to benefit from the reduced heat treatment associated with conventional ODD processes.
The invention is embodied in a method and apparatus for making a multiple overclad optical fiber preform and making optical fiber therefrom. Embodiments of the invention provide a method for making a multiple overclad optical fiber preform, which includes the steps of positioning a first overclad tube around a preform core rod, positioning at least one second overclad tube around the first overclad tube, and collectively heating the preform core, the first overclad tube and the one or more second overclad tubes under pressure to collapse the overclad tubes onto the preform core rod thus producing a multiple-clad optical fiber preform. Heating occurs either in a draw tower furnace, e.g., as part of an Overclad During Draw (ODD) process, or, alternatively, by a separate heating source prior to introducing the preform into the draw tower, e.g., as in a conventional Rod-In-Tube (RIT) process. The preform core rod is positioned into and substantially coaxial with the first overclad tube, e.g., by inserting the distal end of the preform core rod into the proximal end of the first overclad tube and sealing the relative positions thereof. Similarly, the first overclad tube is positioned into and substantially coaxial with the one or more second overclad tubes, e.g., by inserting the distal end of the first overclad tube into the proximal end of the second overclad tubes and sealing their respective positions.
According to embodiments of the invention, a method for making optical fiber from the multiple overclad preform further includes the step of drawing optical fiber as portions of the preform are being heated in the draw tower furnace, as per ODD processes. Alternatively, the multiple overclad preform is heated by a heating source external to the draw tower, e.g., as in conventional RIT processes. The resulting multiple overclad preform then is moved to the draw tower and optical fiber subsequently is drawn therefrom.
According to embodiments of the invention, an apparatus for making a multiple overclad optical fiber preform includes a preform core rod, a first overclad tube surrounding the preform core rod, and at least one second overclad tube surrounding the first overclad tube. Alternatively, the apparatus also includes a quartz disc with or without one or more quartz spacers positioned within at least one of the overclad tubes for supporting the preform core rod and the first overclad tube within the one or more second overclad tubes. The preform rod and the first overclad tube are dimensioned such that the outer diameter of the preform rod is within the range from approximately 0.50 mm to approximately 1.75 mm from the inner diameter of the first overclad tube. Also, the first overclad tube is dimensioned such that its outer diameter is within the range from approximately 0.50 mm to approximately 1.75 mm from the inner diameter of the second overclad tubes.
According to embodiments of the invention, an apparatus for manufacturing a multiple overclad optical fiber preform includes a support for operably positioning an apparatus having a preform core rod and a plurality of overclad tubes, a heat source for heating the apparatus, and a vacuum source for establishing a pressure gradient across the exterior and the interior of the apparatus. Heating the apparatus with the established pressure gradient causes the overclad tubes to collapse onto the preform core rod, thus forming the multiple overclad optical fiber preform. Heating the apparatus occurs in the fiber draw tower, e.g., similar to ODD processes, or, alternatively, prior to the multiple overclad preform being positioned in the draw tower, e.g., similar to conventional RIT processes.